1. Field of the Invention
This invention relates to melamine cyanurate and also to a process for its preparation. More specifically, the present invention is concerned with melamine cyanurate obtained by heating melamine and cyanuric acid without using any liquid medium, especially granular melamine cyanurate and also with a process for its preparation.
2. Description of the Related Art
Halogen-containing flame retardants have heretofore been used as one type of flame retardants for plastics. People have however become concerned with the toxicity of their combustion products to the human body, leading to an increasing demand for the change from low-halogen flame retardants to halogen-free flame retardants. Under these circumstances, substances containing a triazine ring, such as melamine and melamine cyanurate, are now under investigation as potential flame retardants. It is known to be possible to impart flame retardancy to urethane resins or polyamide resins by directly mixing melamine with urethane resins or melamine cyanurate with polyamide resins, both to a proportion of from several percent to 30 percent.
Melamine cyanurate has been prepared generally by separately dissolving melamine and cyanuric acid in water, mixing the resultant aqueous solutions to react them, collecting a precipitate of sparingly-soluble melamine cyanurate so formed, and then drying the melamine cyanurate.
To practice this process on an industrial scale, it is however accompanied inter alia by the following drawbacks: (1) the efficiency of preparation is low because the solubilities of melamine and cyanuric acid in water are too low to prepare thick aqueous concentrations, (2) the reaction product is in the form of fine particles so that its collection by filtration or the like is not easy, and (3) collected melamine cyanurate particles contain substantial water so that a long time is needed for drying them.
Industrial processes are disclosed in Japanese Patent Laid-Open Nos. 55587/1979 and 55588/1979. According to the process disclosed in the former patent publication, the reaction is carried out in the presence of water simply as a medium instead of reacting them in the form of aqueous solutions as described above. To melamine and cyanuric acid which are in the form of solids, respectively, water is added in an amount of 20-2,000 parts by weight per 100 parts by weight of the sum of the reactants, followed by their reaction under mixing or kneading. According to the process disclosed in the latter patent publication, on the other hand, melamine and cyanuric acid each of which has an average particle size not greater than 100 .mu.m are formed into a uniform powder mixture. To the mixture, water is then added in an amount of 30-300% based on the mixture, followed by the reaction. These processes both require the addition of water for conducting the reaction, so that drying and grinding steps are needed subsequent to the reaction.
As a process making use of starting materials different from the above-described processes, Japanese Patent Laid-Open No. 125690/1979 discloses the process that urea and melamine, while still in solid forms, are mixed and heated to prepare melamine cyanurate. This process however requires an expensive corrosion-resistant material for the reactor. Further, when the mixture of urea and melamine is heated, they react with each other in a turbid melt while undergoing deammoniation. Therefore the resulting melamine cyanurate is in the form of a block and is not easy to handle. Its yield is not sufficient and, to obtain a high-purity product, further purification is indispensable.